An LCD has the advantages of portability, low power consumption, and low radiation, and has been widely used in various portable information products such as notebooks, personal digital assistants (PDAs), video cameras and the like. Furthermore, the LCD is considered by many to have the potential to completely replace CRT (cathode ray tube) monitors and televisions.
A typical LCD includes an LCD panel. The LCD panel includes a multiplicity of pixels, each having a capacitance. When a power supply provides an operation voltage to the LCD and then the power supply is turned off, the operation voltage does not immediately decrease. For example, when a power supply voltage of 5V is turned off, a decrease to a residual voltage 0.4 V takes about 20 seconds. If the power supply is turned on again quickly before the residual voltage in the power supply has decreased to a predetermined voltage, this causes an operational error in the LCD. To prevent such operational error, a power switching circuit is provided in the LCD to remove the residual voltage.
FIG. 3 is a circuit diagram of a typical power switching circuit 10 used in an LCD. The power switching circuit 10 includes a control signal input terminal 110 which is configured for receiving control signals, an output terminal 120 connected to the LCD, a twelve volt direct current (DC) power supply 130, a five volt DC power supply 140 functioning as a main power source of the LCD, a first negative-positive-negative (NPN) transistor 150, a second NPN transistor 170, an n-channel enhancement mode metal-oxide-semiconductor (NMOS) transistor 160, a first resistor 155, a second resistor 156, a third resistor 165, a fourth resistor 175, and a fifth resistor 176.
The first NPN transistor 150 includes a base electrode “b” connected to the control signal input terminal 110 via the first resistor 155, an emitter electrode “e” connected to the base electrode “b” via the second resistor 156 and further connected to ground, and a collector electrode “c” connected to the 12V DC power supply 130 via the third resistor 165.
The second NPN transistor 170 includes a base electrode “b” connected to the control signal input terminal 110 via the fourth resistor 175, an emitter electrode “e” connected to ground, and a collector electrode “c” connected to the output terminal 120 via the fifth resistor 176.
The NMOS transistor 160 includes a gate electrode “G” connected to the collector electrode “c” of the first NPN transistor 150, a source electrode “S” connected to the output terminal 120, and a drain electrode “D” connected to the 5V DC power supply 140.
In order to apply a 5V voltage from the 5V DC power supply 140 to the output terminal 120, a first control signal such as a low level 0V voltage is provided to the control signal input terminal 110 by an external circuit (not shown). Thus the first NPN transistor 150 and the second NPN transistor 170 are switched off. A 12V voltage from the 12V DC power supply 130 is applied to the gate electrode “G” of the NMOS transistor 160 via the third resistor 165. Thus the NMOS transistor 160 is switched on, and the 5V voltage from the 5V DC power supply 140 is applied to the output terminal 120 via the activated NMOS transistor 160.
In order to suspend the supply of the 5V voltage from the 5V DC power supply 140 to the output terminal 120, a second control signal such as a high level 5V voltage is provided to the control signal input terminal 110 by the external circuit. Thus the first NPN transistor 150 and the second NPN transistor 170 are switched on. The gate electrode “G” of the NMOS transistor 160 is connected to ground via the activated first NPN transistor 150, so that the NMOS transistor 160 is switched off. Thus, the 5V voltage from the 5V DC power supply 140 cannot be provided to the output terminal 120. Electric charges stored in the LCD which is connected to the output terminal 120 can be discharged quickly through the actived second NPN transistor 170.
Referring to FIG. 4, a current wave diagram of the power switching circuit 10 is shown. When the NMOS transistor 160 is switched on, and the supply of the 5V voltage is provided to the LCD via the activated NMOS transistor 160, a five amperes rush current is generated at the moment that the NMOS transistor 160 is switched on. The rush current may accelerate an aging process of electronic devices of the LCD. Thus a service life of the LCD is reduced.
It is desired to provide a new power switching circuit used in an LCD which can overcome the above-described deficiencies.